Binary Colloidal Alloy Test – 5 (BCAT-5)
The Binary Colloidal Alloy Test – 5 (BCAT-5) hardware supports four investigations. Samples 1 – 5, the Binary Colloidal Alloy Test – 5: Phase Separation (BCAT-5-PhaseSep) will study collapse (phase separation rates that impact product shelf-life). In microgravity the physics of collapse is not masked by being reduced to a simple top and bottom phase as it is on Earth. Samples 6 – 8, Binary Colloidal Alloy Test – 5: Compete (BCAT-5-Compete) will study the competition between phase separation and crystallization, which is important in the manufacture of plastics and other materials. Sample 9, Binary Colloidal Alloy Test – 5: Seeded Growth (BCAT-5-SeededGrowth) will study the properties of concentrated systems of small particles when 99.8% are identical 0.36 diameter micron spheres and 0.2% are 4.14 microns in diameter (11.5x larger); these seed particles may cause heterogeneous crystal growth. Sample 10, Binary Colloidal Alloy Test – 5: Three-Dimensional Melt (BCAT-5-3D-Melt) will look at the mechanisms of crystal formation and 3-dimensional melting using colloidal particles that change size with temperature.
The BCAT-5-3D-Melt sample will consist of a monodisperse nearly-hard-sphere colloidal suspension near its crystallization point. For this investigation small temperature changes that change the particle volume fraction will move the equilibrium system towards and away from the melting transition. The key ingredient in these samples is a thermosensitive polymer, poly(N-isopropylacrylamide (NIPA); the temperature-sensitive character of the samples stems from the temperature dependent solubility of NIPA polymer in water. Below its theta temperature (the temperature at which the coiled polymer molecules expand to their full contour lengths and become rod-shaped) of approximately 31 degrees C, water is a good solvent and NIPA polymer assumes a swollen coil form; in this regime a small increase of temperature increases monomer-monomer attractions and thus the size of the isolated polymer decreases. Above the theta temperature, water is a poor solvent and NIPA has a collapsed globule form.
The BCAT-5-3D-Melt experiment will simply record sample temperature and observe (by photography) whether the sample crystallizes, noting at what temperature the sample experiences the fluid-solid transition. This information will be also be useful for future experiments planned for ISS.